#3470594 Refine dragging behavior and design in Layers menu

Closes #3470594

GIF is in the issue summary

Changes:

• You can now drag into an empty slot.
• Added border bottom blue as a ghost element which matches the preview ghost element. Also the border bottom dynamically changes width based on the depth of the layer which helps differentiate between dragging into an SDC's slot vs. dragging into the root level.
• You no longer can drag things into an SDC as siblings of their slots.
• Clone element also taken from preview.tsx where the clone element is the same element but with a lower opacity.
• Changed the Slot label from the machine name to just 'Slot'.
• Changed hovered element to be a gray background, selected element has a blue background.
• Early return when an element is drag and dropped into the same position so no unnecessary redux calls are made.